1. Field of the Invention
This invention relates to an interrogator circuit for use in a tagging system which has tags including a semi-passive or reflective modulating transponder.
2. Description of the Related Art
The application of tagging systems using transponder circuits is becoming increasingly widespread. Tagging systems generally comprise a number of integrator circuits which are connected to a control centre by means of a communications network and a number of tags with which the interrogator can communicate typically by means of radio links. Generally an interrogator circuit is at a fixed location whilst the tags are fitted to objects which are mobile.
One example of a tagging system is a road tolling system in which vehicles are fitted with a transponder (tag) which can be interrogated by interrogating circuits which are situated at toll collection points around the road network. Communication between the vehicle and the interrogating circuit provides the requisite tolling information for charging the road user. The applications for tagging systems are virtually limitless, for example in the field of telematics, that is, the communication between an infrastructure and vehicles, it has been proposed to use such tags in the translation of road signs, for routing information and bus priority schemes to name but a few. Other applications include logistics, for example keeping tack of the movements of goods such as food between a warehouse and a number of retail outlets. In all of these applications the transponder circuit which typically operates at microwave frequencies, needs to be inexpensive and have a long opening life. To meet the former require the circuitry to be simple, whilst the latter generally requires the circuit to have low power consumption and be capable of operating from a battery source.
Transponder circuits for tags may be "active", that is it includes its own radio transmitter and is able to transmit without the assistance of the interrogator circuit, purely "passive" that is they can be read by an interrogator but cannot themselves talk to the interrogator circuit, or be what is termed semi-passive. Semi-passive transponders can detect data transmitted to them from the interrogator and transmit information to the interrogator by reflecting and modulating the signal received from the interrogator. As such, semi-passive transponders do not include an active transmitter and rely solely on the interrogator to provide the communication medium. Such transponders are also often referred to as reflective modulator type transponders. Since such transponders require minimal radio frequency circuitry the tags are compact and have very low power consumption making them ideally suited to many tagging applications. The present invention relates to an interrogator circuit which is suitable for use with semi-passive transponders. This requires the interrogator circuit to be cable of transmitting a signal and simultaneously demodulating a received signal.
The suitability of tagging systems to a given application is at present largely limited by the cost of the interrogator circuit and tag transponder. Whilst much time and effort has been expended in developing efficient, inexpensive semi-active tag transponders the interrogator circuits remain relatively complex and expensive. Usually a separate receiver and transmitter circuit are provided which may or may not use a common antenna. Often these circuits include double balanced mixers which are configured as an image reject front end and have I and Q baseband receivers. Whilst such interrogator circuits are found to work very effectively in terms of range and robustness they have an associated problem that their production cost is high, typically of the order of a few hundred pounds. A number of applications exist for a much lower cost interrogator circuit which is capable of operating at a moderate range, typically ten metres or so. In addition to the need for an inexpensive interrogator, a whole new range of applications arises if the interrogator circuit could be operated from a battery source. At present, whilst the tags have very low power consumption, the interrogator, which is typically located at a fixed location and has the benefit of a mains power supply available, has a relatively high power consumption. A need therefore exists for an interrogator circuit for use with semi-passive transponders which has very low power consumption and can be effectively operated from a battery supply over a long period of time.